Several reports indicate that early osteolytic activity is needed for prostate cancer (PCa) bone metastases to become established but eventually PCa switches to an osteoblastic phenotype through unknown mechanisms. Wnts promote skeletal formation, osteoblast differentiation and bone remodeling. Accordingly, we explored if altered Wnt signaling in the bone microenvironment impacts the bone metastatic phenotype. We found that PCa cells express Wnt and Wnt inhibitor DKK-1 and that Wnts contribute to PCa's ability to nduce osteoblastic activity. Furthermore, DKK-1 inhibited PCa-osteoblastic activity and DKK-1 decreases in PCa bone metastases. Accordingly, we hypothesize that DKK-1 acts as a molecular swith that transitions PCacells from osteolytic (high DKK-1 expression and Wnt inhibition) to osteoblastic (low DKK-1 expression and permissive for Wnt) activity as the PCa progresses in the bone. To test this, we will perform the following Aims. Aim 1. Define the role of temporal downregulation of DKK-1 in PCa bone metastases. We will perform in vivo murine studies using an inducible promter driving DKK-1 cDNA or anti-DKK-1 antibody to temporally increase or decrease DKK-1 activity, respectively, in early and late stages of PCa bone metastases. We will then evaluate effects on tumor establishment and progression in bone and the bone phenotype. Aim 2. Determine the mechanism(s) through which DKK-1 impacts PCa-induced bone remodeling. We will determine (1) how modulation of DKK-1 in PCa cells affects their ability to induce osteoblast differentiation and (2) how DKK-1, in the context of PCa, alters osteoblasts ability to interact with other bone remodeling pathways including BMPs, VEGF, ET-1 and RANKL. This will be done through a series of in vitro experiments using osteoblast cell lines and primary osteoblasts. Aim 3. Identify mechanisms that regulate DKK-1 expression in bone metastatic PCa cells. We have found that parathyroid hormone-related protein (PTHrP) decreases DKK-1 expression. We will extend these studies to determine the ability of PHTrP to regulate DKK-1 expression at the protein, mRNA and promoter levels and determine the mechanism through which PTHrP regulates DKK-1 promoter activity in PCa and determine how this impacts the PCa bone metastatic phenotype in vivo. When completed, this proposal will have elucidated if temporal regulation of DKK-1 expression accounts for the ability of PCa skeletal metastases to transition from an osteolytic to an osteoblastic phenotype and the mechanisms through which DKK-1 is switched off and how that modulates bone remodeling in PCa bone metastases. RELEVANCE (See instructions): Spread of prostate cancer (PCa) to bone is a common and painful complication of PCa. Understanding the mechanisms of how PCa interacts in the bone will help identify therapies to inhibit this frequent complication of PCa.